Thermal transfer processes are well known in applications such as color proofing as a means of dry transferring or printing of dye and/or pigment layers. Such thermal transfer processes typically use a laser to induce the image-wise thermal transfer of material.
Laser-induced thermal transfer processes typically use a donor element, including a layer of material to be transferred, referred to herein as a transfer layer, and a receiver element, including a surface for receiving the transferred material. Either the substrate of the donor element or the receiver element is transparent, or both are transparent. The donor element and receiver element are brought into close proximity or into contact with each other and selectively exposed to laser radiation, usually by an infrared laser. Heat is generated in the exposed portions of the transfer layer, causing the transfer of those portions of the transfer layer onto the surface of the receiver element. If the material of the transfer layer does not absorb the incoming laser radiation, the donor element should include a heating layer, also known as a light-to-heat conversion (LTHC) layer or a transfer-assist layer, in addition to the transfer layer.
In a typical laser-induced digital thermal transfer process the exposure takes place only in a small, selected region of the assembly at a time, so that transfer of material from the donor element to the receiver element can be built up one region at a time. The region may be a pixel, some portion of a pixel or a plurality of pixels. Computer control facilitates the transfer at high speed and high resolution. Alternatively, in an analog process, the entire assembly is irradiated and a mask is used to selectively expose desired portions of the thermally imageable layer.
A particular need for printable electronics includes thermally imageable insulating layers or dielectric passivation layers. WO 2005/004205, for instance, discloses a method of forming a pattern of filled dielectric material on a substrate by a thermal transfer process comprising exposing to heat a thermally imageable donor element comprising a base film, a light to heat conversion (LTHC) layer, and a transfer layer of dielectric material. In a thin film transistor (TFT), the dielectric layer serves to insulate the gate layer from the semiconductor and source-drain layers. Its primary function is to allow the passage of fields, but not currents. A fundamental requirement is that the dielectric layer possess high volume resistivity, greater than 1014 ohm-cm, to prevent leakage currents; and be largely pinhole free to prevent catastrophic shorts between conductive layers. The dielectric layer also must have high purity in order not to dope the adjacent semiconductor layer; it should be thin, for instance, about 5 microns or less, and have a high dielectric constant for low-voltage operation.
Additional requirements for successful thermal transfer of the dielectric layer include: the dielectric composition must be coatable and therefore must have adequate solubility and/or dispersability in a suitable solvent; it must exhibit good interfacial behavior (mechanical, electrical) with adjacent layers, including the receiver, the conducting layers (gate and source-drain layers) and the semiconductor layer; and it must be printable by thermal transfer and maintain its insulating properties. In order for an insulating layer to meet this last requirement and be printed in one printing cycle, it needs to exhibit good adhesion to all previous layers including receiver, conducting layers and semiconductor layers. It must print under approximately the same conditions (e.g., same drum speed and power) onto all of the different previous layers, and it must print with acceptable quality. For example, defects caused by the thermal transfer process including: pinholes, bubbles, cohesive failure, breaks at swath boundaries, and co-transfer of materials from the adjacent donor substrate layers (typically the LTHC layer); must be very minimal in order not to degrade electrical performance to an unacceptable level.
There is a need for thermal transfer donors that allow patterned thermal transfer of dielectric layers with excellent resistivity that exhibit good transfer properties and good adhesion to a variety of materials. Particularly desirable are thermal transfer donors wherein, after transfer, the patterned layer has the uniformity, continuity and resistive properties required of a dielectric layer in electrical applications, for instance in applications for capacitors or thin film transistors.